1. Field of the Invention
This invention relates to a variable focal length lens for low magnification, which is to be used as a projecting lens for finite distances in a photograph enlarging apparatus, or the like.
2. Description of the Prior Art
Various photograph enlarging apparatuses, with which images recorded on negative films are printed on sheets of photographic paper, have heretofore been known. In photograph enlarging apparatuses for general use, a lens having a fixed focal length is used as a projecting lens, and the relative distances among the negative film the lens, and the photographic paper are changed respectively. On the other hand, in photograph enlarging apparatuses for business use, a variable focal length lens is often used as a projecting lens, and the positions of the negative film and the photographic paper are fixed.
As the variable focal length lens to be used in photograph enlarging apparatuses for business use, there has been known a lens composed of an ordinary fixed focal length lens and a single lens referred to as a diopter lens, which are combined with each other in an appropriate manner. The manner, in which the fixed focal length lens and the diopter lens are combined with each other, is changed in various ways, and the focal length is thereby changed in order to provide a predetermined magnification.
However, in cases where the variable focal length lens having the structure described above is used and the focal length is to be changed largely, the combination of the diopter lens becomes complicated. Therefore, the image quality of the resulting printed image inevitably becomes bad. Also, when a desired magnification is set, a long time is required for the positions of the lenses to be adjusted.
In view of the above circumstances, a novel variable focal length lens has been proposed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-129613. In the proposed variable focal length lens, a lens system is constituted of two sets of lenses comprising a negative set lens and a positive set lens, and the focal length is changed by changing the distance between the two sets of lenses.
With the proposed variable focal length lens, the image quality of the printed image can be kept good, and the lens cost can be kept low because no diopter lens is required.
However, with the above-described conventional variable focal length lens comprising the two sets of lenses, the distance between the lens and the negative film (i.e., the finite back focal length) becomes as short as, for example, 90 mm or less at the time of a high magnification. If the finite back focal length is thus short, the operator cannot directly view the negative film due to a photometric mirror, which is inserted for color correction between the lens and the negative film. Therefore, the problem occurs in that the operator cannot easily carry out the printing work.